Paper manufacture



Dec. 8, 1959 Filed April 15. 1957 J. A. HARPHAM PAPER MANUFACTURE 2Sheets-Sheet 1 JOHN A. HAR PHAM INVENTOR.

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AGENT Dec 8, 1959 J. A. HARPHAM PAPER MANUFACTURE 2 Sheets-Sheet 2 FiledApril l5, 1957 .md 2.o of@ mnd @no @No @No 2.o 2.o @0.o

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BY vl-Maf www AGENT `United States Patent() PAPER MANUFACTURE John A.Harpham, Wilmington, Del., assignor to Hercules Powder Company,Wilmington, Del., a corporation of Delaware Application April 15, 1957,Serial No. 652,898

12 Claims. (Cl. 162-157) purification and either with or withoutmechanical puri` fication. The term purified with reference to cottonlinters or linters is used herein to designate raw cotton linters whichhave been chemically purified by conventional techniques Which are wellknown in the art, and

whenever' mechanical purification alone is meant, that will be sospecified. Degree of substitution (D.S.) is the number of hydroxyalkylgroups per anhydroglucose unit of cellulose.

One of the important properties of paper is its strength as measured byvarious tests such as Mullen burst, Elmendorf tear, fold strength,tensile strength, and the like. Heretofore, difficulty has beenexperienced in using purified cotton linters yas a feed in commercialmanus facture of fine papers, such as writings, bonds, ledgers,blueprints, and the like, one of the reasons being the failure ofpurified cotton linters to develop sufficient strength under the usualconditions of treatment. The strength properties of the purified cottonlinters can be developed by unusual conditions of treatment in beating,jordaning, and other similar operations, but such unusual treatments areuneconomical and impractical in commercial operations.

In general, there is a desire in the paper industry for improvedstrength levels in paper from all types of furnishes, and the paperindustry is endeavoring to improve these levels by such means asinternal additives, wet and dry strength resins, and the like.

In accordance with this invention, it has been found that the strengthproperties 0f paper can be substantially improved by employing as a feedin the preparation of paper raw cotton linters which have beenchemically modified and then purified. The chemical modification of thelinters adds hydroxyalkyl substituent groups to the anhydroglucose unitin such anamount that the chemically modiried linters retainsubstantially the fibrous form of unmodified linters and are bothWater-insoluble and alkali-insoluble. Purification is by well-knowntechniques which will be disclosed more fully hereinafter.

A unique, very important and surprising feature of this invention is thediscovery that the specific chemical modification of the raw linterswhich adds hydroxyalkyl substituent groups provides properties in thepaper not obtainable with other chemical modifications of the rawlinters, such as 'carboxymethylation Thus, introduction of hydroxyalkylsubstituents, for example, hydroxyethyl substituent, provides animprovement in paper strength properties such as burst, fold, tensileand tear not obtainable with carboxymethyl substitution.

Furthermore,

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an additional yand significant distinction is also observed in thesizing of the chemically modified cotton linters in the formation ofsized paper. The improved strength properties resulting from chemicalmodification of the linters paper furnish by hydroxyalkylation carryover into the sized paper to a larger extent, making it possible toproduce sized paper of still greater strength properties than it ispossible to obtain from other chemically modified linters paperfurnishes, such as carboxymethylated furnish. This latter distinction issignificant in the use of the hydroxyalkyl-substituted linters in theproduction of sized paper.

In the art of paper making, it is common knowledge that the levels ofcertain paper properties, eg., tear strength, opacity, and porosity, arelowered during refining. Tear strength actually increases during theinitial stages of refining, but in the commercial range of refining thetear 'strength level is decreasing; the levels of other paperproperties, e.g., fold strength, burst strength, tensile strength, anddensity, are increasing. The rates at which these various propertieschange during refining are substantially fixed for a given cellulosefurnish. However, the same furnish, chemically modified in accordancewith this invention, shows a disproportionation of the rates at whichthe various properties develop such that the end result is a paper withan entirely different cornbination of properties.

Improvements in desirable paper properties other than those discussedy'above are realized by practicing this invention. While theseadditional properties are highly desirable, no standard means has beenestablished by the industry for measuring them. For example, in the finepaper industry the stiffness, surface and feel of the sheet areimportant. Heretofore, when attempting to use unmodified cotton linters,the industry has been unable to obtain a high stiffness, the type ofsurface and the feel 0f the sheet which it regards as beingindispensable. The present invention makes it possible to overcome thesedrawbacks.

Advantages to be gained by practicing this invention are demonstrated bythe examples below. In examples l to 8, all the samples of eitherunmodified and purified cotton linters or chemically modified andpurified cotton linters were refined at a 2.5% consistency (neutral pH)in a 1% lb. Valley laboratory beater for 2.5 hoursl with a 6-kg.bedplate load. In all the examples the refined samples were handsheetedusing Noble and Wood handsheeting equipment to a 40 lb. basis weight (24x36- 500) and the handsheets were then conditioned and tested bystandard TAPPI procedures (Technical Association of the Pulp and PaperIndustry). These tests included Mullen burst, Elmendorf tear, tensilestrengths and MIT folding endurance. The strength data were converted toa 40 lb. basis weight using the linear relationships between the variousstrengths and basis weight. Percent as used herein is on a weight basis.

.EXAMPLE 1 A sample of second-cut cotton linters that had beenmechanically and chemically purified was prepared into handsheets in themanner described above. The purification was conventional and includeddigesting in caustic at an elevated temperature `and pressure andbleaching to give a high brightness cotton cellulose pulp. This examplerepresents a control run, the data of which are to be compared with thedata of Examples 2-8 listed in Table I hereinafter. Y A

EXAMPLES 2, 3 AND 5 Three samples of raw mechanicallypurifiedrsecond-cut cotton linters were steeped in aqueous caustic,reacted with gaseous ethylene oxide, and then cooked and bleached' 3 asin Example l above. The resulting linters samples, having hydroxyethylD.S. of v0.05, 0.1 and 0.2 (shown as HEC), were then handsheeted andtested in the manner described above. The 4data are listed in Table Ibelow.

EXAMPLE 4 A sample of raw, mechanically purified second-,cut cottonlinters was steeped in aqueous caustic, reacted with gaseous ethyleneoxide, and then cooked and bleached as modified and purified cottonlinters, prepared from mechanically purified raw cotton linters as inthe preceding examples, were refined for two hours in a cycle beater at3.6% consistency with 1GO-lb. roll pressure. In all the examples, thesamples were removed and treated with size. Then the sized samples werehandsheeted, tested, and the paper strength data converted to a 40-lb.basis weight, all as described above for Examples l-8. By percent sizeas shown in Table II hereinafter is meant the amount of IhEIXIDHeO'B1(TISU1I1S lill-lflel) having a: hYdTOXX- l0 size started with andnot the amount retained on the e y o s own as were tiieril'iannhamisheetg sheeted and tested in the manner described above. TheEXAMPLE 9 data are listed in Table -I. This example is the same asExample 7 of my application, Serial No. 503,803, filed A Sample ofSecond'ut cotton-hnters that had been April 25, 1955, and my copendingapplication, Serial No. mechanically and diemlcauy plmfd as m Example I587,471, med May 28 1956 except that thestrength data above was treatedwith gum rosin size, then handsheeted listed in Table I hereinafter havebeen converted to a 40- and tested' Thls example represents. a controlrun the lb. basis weight for comparison purposes. data of which 'fue to.be compared with the data of EX' amples 10-13 listed in Table IIhereinafter. A l f EXAMPIIIE 6 l zo EXAMPLES 10 AND 11 samp e o rawmechanica y pur' ed second-cut cotton linters was steeped in aqueouscaustic, reacted with csamls (if hydsiyetllged cotto? hlftsll propyleneoxide, and then cooked and bleached as in ast ant glmg a '.0 t/erellzedted! el Example 1. The resulting linters, having ahydroxypropyl stemounS gum msm Slze en an s ee an D.S. of 0.05 (shown as HPC), were thenhandsheeted and tested in the manner described above. The data arelisted EXAMPLES 12 AND 13 in Table I. This example is the same asExample 6 of my These examples were similar to Examples 10 and 11copending application, Serial No. 534,881 filed September with theexception that an alkyl ketene dimer (Aquapel 16, 1955, and it is thesame as Example 9 of my applicaas marketed by Hercules Power Company,Wilmington, tion, Serial No. 503,803, filed April 25, 1955, except that30 Delaware) was employed as the sizing material.

' Table 11 Example 9 Example 10 Example 11 Example 12 Example 13Paper-Finnish unmodified Purified HEC HEC HEC.. HEC.

Cotton Linters. D.s 0.05 0.05 0.05 0.05. lzahgi slum Rosin. 2Guin Rosin.luapel.. (.tiqunpel Mefen BmS'iTrJE-.iijj Elmendorf Tear, g./sheet MITFolds, double the strength data listed in Table I hereinafter have beenconverted to a 40-lb. basis weight for comparison purposes.

EXAMPLE 7 A sample of raw mechanically purified second-cut cottonlinters was steeped in aqueous caustic, reacted with butylene oxide, andthen cooked and bleached as in Example l. The resulting linters, havinga hydroxybutyl D.S. of 0.01 (shown as HBC), were then handsheeted andtested in the manner described above. The data are shown in Table I.

EXAMPLE 8 A sample of mechanically purified second-cut raw cot- Table lThe advantages of this invention are realized by chemically modifyingcellulose in such a manner that hydroxyalkyl substituent or group isintroduced to the anhydroglucose units and employing the resultingchemically modified cellulose, having a low degree of substitution, inpaper manufacture. Among the hydroxyalkyl substituents that areintroduced to the cellulose are the hydroxyethyl, hydroxypropyl,hydroxybutyl, and similar radicals or groups, either singly or incombination.

For the practice of this invention, the degree of substitution of thehydroxyalkyl cellulose is such that the cellulose derivative iswater-insoluble and alkali-insoluble, and the chemically modifiedcellulose retains substantially the fibrous form of the unmodifiedcellulose. The actual degrees of substitution necessary to impart eitheralkali solubility or water solubility to a chemically modified celluloseare either known or easily determinable and depend to some extent uponthe method of preparation of the modified cellulose and upon theuniformity of the substitution.

In the practice of this invention, it has been found Example 1 ExagipleExample 3 Example Example Example Example 5 6 7 8 Unmodled Purled CottonLinters.

Paper Furnlsh D.S Mullen Burst, lb./sq. in

MIT Folds, double Tensile. lb./in

In the following Examples 9-13 all the ,samples of either unmodifiedandpurified cottonlnters or Acheiriically HEO BEPC necessary to employ ahydroxyethyl cellulose having a degree of substitution not in excess ofabout 0.20. There are a number of reasons for this. One reason for thisrestriction as to D.S. is that the higher the D S.v the greater thehydrophilic nature of the linters and as a consequence, under conditionsof high humidity, the increased hydrophilic nature is apt to cause adecrease in the improved stiffness obtained by chemical modification.Another consequence of the increased hydrophilic nature is that itresultsin production problems such as difficulty in drying the modifiedlinters and excessive shrinking of the paper sheet during drying.Another reason for this restriction as to D.S. is that D.S and papersheet opacity vary inversely. Above a D.S. of about 0.20 the decrease inopacity has become quite undesirable. A further reason for the D.S.restriction is that, based on applicants finding, the over-al1 strengthproperties of the paper level off at a D.S. of about 0.20.

Thus by operating at a D.S. not exceeding about 0.20, applicant avoidsthe difficulties discussed above and still obtains a paper havingsubstantially maximum strengths.

In considering the strength of paper it is the over-all strengthproperties, rather than an individual strength property, which areimportant. Such a measure of the over-all strength properties of paper,as disclosed above, has been referred to in the art and trade as thestrength number and expressed as` 3V Tear burst X log fold see TAPPI 33,370 (1950).

Figs. l-4 of the accompanying drawing clearly show that the over-allstrength properties of paper level off at a D.S. of about 0.20. Data forFigs. 1-3 were obtained by preparing and testing five samples, onesample in exactly the same manner as Example 1 hereinbefore (i.e., anunmodified cotton linters control) and four samples in exactly the samemanner as Examples 2, 3 and 5 hereinbefore, except for using beatingtimes of 2 and 2.5 hours and varying amounts of ethylene oxide` to giveD.S. levels of 0.11, 0.21, 0.28 and 0.43. Thus, the unmodified and themodified linters were refined and portions thereof were removed from thebeater after 2.0 and 2.5 hours beating and handsheeted. The handsheetproperties of tear, burst and fold Werevmeasured according to TAPPImethods. These properties are shown graphically in Figs. 1-3, D.S.having been plotted against tear in Fig. l, against burst in Fig. 2, andagainst fold in Fig. 3. From Figs. l-3 tear, burst, and fold valuesrespectively were read off the graphs at each beating time (2 and 2.5hours) for the D.S. values of 0, 0.10, 0.20, 0.30 and 0.40. These tear,burst and nfold values were then used to determine the strength numberof the paper at all of these latter D.S. values for each beating timeaccording to the formula Va Tear burst X log fold and these strengthnumbers Were plotted graphically against these latter D.S. values inFig. 4. It is clear from Fig. 4 that the "strength number levels ofi ata D.S. of about 0.20.

Thus by practicing this invention one obtains an improved paper havingsubstantially increased over-all strength properties (Fig. 4) withoutencountering production problems and without a sacrifice in otherdesirable properties of paper such as high stiffness and opacity.

The chemically modified cellulose for use in the practice of thisinvention is usually formed by first converting mechanically purifiedraw cotton linters to alkali cellulose by steeping in an aqueous causticsolution. The alkali cellulose is then reacted with a suitablehydroxyalkylating agent, such as ethylene oxide, in order to introducethe desired amount of hydroxyalkyl substituent into the anhydroglucoseunit. The reaction can be carried out in a slurry medium, such as anaqueous solution of a lower aliphatic alcohol, for example, propanol,isopropanol, tertiary butanol, and the like, but if desired vtheslurrying medium need not be used.

To form the improved paper product of this invention, the chemicallymodified cellulose is made into an aqueous suspension, and commerciallythis suspension usually has a consistency not greater than about 5%. Theaqueous suspension can then be passed to a suitable beater, and it willusually be found that the strength properties of the paper productdepend to a substantial extent upon the length of the beating cycle. Itis preferred that the beating cycle not exceed a period of several hoursfor economical commercial operation. Actually, if given an adequatebeating'period, unmodified purified secondcut cotton linters can be usedto form a paper having satisfactory properties for many purposes.However, in order to impart satisfactory strength to such papers, it isnecessary to provide the linters With an unusual and uneconomicalrefining treatment. By practicing this invention the strengthdevelopment with chemically modified second-cut cotton linters can beachieved in conventional retining cycles. After beating, the aqueoussuspension is used to form paper in any of the conventional procedures.For example, the aqueous suspension can be continuously flowed onto atraveling endless tinemeshed screen called a Wire, and water is drainedfrom the suspension through the wire. The chemically modified celluloseforms a matted or felted web of wet fibers on the wire from whichadditional water is removed by suction. The web is then carried throughseveral pairs of press rolls to squeeze out as much additional water aspossible. Most of the remaining Water is subsequently removed byevaporation as the wet web is pressed against successive smooth surfacesof revolving steam-heated cylinders or drums. The dried web usually isnext passed between calender rolls for smoothing and then reeled up.

Paper produced in accordance with this invention can be givenconventional treatments used in paper manufacture, such as addition ofvarious modifying materials, for example, size, coloring materials andmineral fillers. A satisfactory method of introducing these modifyingmaterials involves their addition to the aqueous suspension ofchemically modified cellulose prior to its passage onto the travelingwire, for example, during the beating operation.

In practicing this invention, the chemically modified cellulose, such ashydroxyethyl cellulose, can be used alone or in admixture with othercellulosic materials for the preparation of paper by conventionalrefining techniques having the desired strength properties.

While the chemically modified cellulose herein disclosed is particularlysuitable for the production of fine papers such as bonds, ledgers,blueprints and the like it may also be used in the production of othertypes of papers. Thus, by changing refining procedures'as, for example,by shortening the refining time, it is possible to obtain paper havingthe requisite porosity, absorbency and strength to adapt it for use infilter paper, battery separator plates, laminates and the like. Suchpapers, compared to those produced from cellulose which has not beenchemically modified as herein described, are considerably stronger atthe same porosity levels and considerably more porous at the samestrength levels.

This application is a continuation-in-part of my copending applications,Serial No. 534,881, filed September 16, 1955, and Serial No. 587,471,filed May 25, 1956, which in turn are continuation-impart of myapplication, Serial No. 503,803, led April 25, 1955, the latter threenow abandoned.

What I claim and desire to protect by Letters Patent it:

1. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water-insoluble and alkali-insoluble hydroxyalkylcellulose whereby a paper of improved strength is obtained, saidcelluose resulting from etherifying raw cotton linters to a D.S. not inexcess of about 0.20 and then chemically purifying same.

2. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which Acomprises employing as afibrous material a water-insoluble and alkali-insoluble hydroxyethylcellulose whereby a paper .of improved strength is obtained, saidcellulose resulting from etherifying raw cotton linters to a D.S. not inexcess of about 0.20 and then chemically purifying same.

3. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water-insoluble and alkali-insoluble hydroxypropylcellulose whereby a paper of improved strength is obtained, saidcellulose resulting from etherfying raw cotton linters to a D.S. not inexcess of about 0.20 and then chemically purifying same.

4. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water-insoluble and alkali-insoluble hydroxybutylcellulose whereby a paper of improved strength is obtained, saidcellulose resulting from etherifying raw cotton linters to a D.S. not inexcess of about 0.20 and then chemically purifying same.

5. In the method for forming paper wherein an aqueous suspension of afibrous material is dewatered on a screen to form a matted web and saidmatted web is dried, the improvement which comprises employing as afibrous material a water-insoluble and alkali-insoluble hydroxyethylhydroxypropyl cellulose whereby a paper of improved strength isobtained, said cellulose resulting from etherfying raw cotton linters toa D.S. not in excess of about 0.20 and then chemically purifying same.

6. As a new article of manufacture, a sheeted paper product of improvedstrength comprising fibrous raw cotton linters modified chemically withhydroxyalkyl substituent and then chemically purified such that themodified cellulose is water-insoluble, alkali-insoluble, has a D.S. notin excess of about 0.20 and retains substantially the fibrous structureof the unmodified cellulose.

7. As a new article of manufacture, a sheeted paper product of improvedstrength comprising fibrous raw cotton linters modified chemically withhydroxyethyl substituent and then `chemically purified such that themodified cellulose is water-insoluble, alkali-insoluble, has a D S. notin excess of about 0.20 and retains substantially the fibrous structureof the unmodified cellulose.

8. As a new article of manufacture, a sheeted paper product of improvedstrength `comprising fibrous raw cotton linters modifiedl chemicallywith hydroxypropyl substituent and kthen chemically purified such thatthe modified cellulose `is Water-insoluble, alkali-insoluble, has laD.S. not in excess ,of `about 0.20 and retains substantially the fibrousstructure of the unmodified cellulose.

9. As a new article of manufacture, a sheeted paper product of improvedstrength comprising fibrous raw cotton linters modified withhydroxybutyl substituent and then chemically purified such that themodified cellulose is water-insoluble, alkali-insoluble, has a D S. notin excess of about 0.20 `and retains ,substantially the fibrousstructures of the unmodified cellulose.

10. As a new article o f manufacture, a sheeted paper product ofimproved strength comprising fibrous raw cotton linters modified withhydroxyethyl and hydroxypropyl substituents and then chemically purifiedsuch that the modified cellulose is water-insoluble, alkaliinsoluble,has a D.S. not in excess of about 0.20 and retains substantially thefibrous structure of the unmodified cellulose.

1l. As a new article of manufacture, a sheeted paper product of improvedstrength comprising fibrous raw cotton linters modified chemically withhydroxyalkyl substituent and then chemically purified such that themodified cellulose s water-insoluble, alkali-insoluble, has a D.S. ofnot in excess of about ,0.20 and substantially retains the fibrousstructure of the unmodified cellulose, said paper product being sizedwith gum rosin size.

12. As a new article of manufacture, a sheeted paper product of improvedstrength comprising fibrous raw cotton linters modified chemically withhydroxyalkyl substituent and then chemically purified such that themodified cellulose is water-insoluble, alkali-insoluble, has a D.S. ofnot in excess of about 0.20 and substantially retains the fibrousstructure of the unmodified cellulose, said paper product being sizedwith an alkyl ketene dimer.

References Cited in the le of this patent UNITED STATES PATENTS1,941,278 Schorger Dec. 26, 1933 2,038,679 Richter Apr. 28, 19362,172,109 Reichel et al. Sept. 5, 1939 2,190,445 Ellsworth Feb. 13, 19402,268,112 Dreshfield Dec. 30, 1941 2,533,145 Schorger Dec. 5, 19502,535,690 Miller et al. Dec. 26, 1950 2,627,477 Downey Feb. 3, 1953FOREIGN PATENTS 674,577 Great Britain June 25, 1952

1. IN THE METHOD FOR FORMING PAPER WHEREIN AN AQUEOUS SUSPENSION OF AFIBROUS MATERIAL IS DEWATERED ON A SCREEN TO FORM A MATTED WEB AND SAIDMATTED WEB IS DRIED, THE IMPROVEMENT WHICH COMPRISES EMPLOYING AS AFIBROUS MATERIAL A WATER-INSOLUBLE AND ALKALI-INSOLUBLE HYDROXYALKYLCELLULOSE WHEREBY A PAPER OF IMPROVED STRENGTH IS OBTAINED, SAIDCELLULOSE RESULTING FROM ETHERIFYING RAW COTTON LINTERS TO A D.S. NOT INEXCESS OF ABOUT 0.20 AND THEN CHEMICALLY PURIFYING SAME.